A comparison of plasma prolactin levels in young female Long-Evans and Holtzman rats as measured by Nb2 lymphoma bioassay and radioimmunoassay

Effects of prolactin on expression of Fos-related antigens in tyrosine hydroxylase-immunoreactive neurons in subdivisions of the arcuate nucleus

Dual immunohistochemistry was employed to determine the effects of prolactin on expression of Fos and its related antigens (FRA) in tuberoinfundibular dopamine (TIDA) neurons located in the dorsomedial (DM) and ventrolateral (VL) subdivisions of the arcuate nucleus (ARC) in the male rat. Systemic administration of the DA receptor antagonist haloperidol caused a sustained (up to 12 h) increase in plasma prolactin concentrations that was accompanied by a transient increase (at 3 h) in the percentage of tyrosine hydroxylase (TH)-immunoreactive (IR) neurons containing FRA-IR nuclei in the DM-ARC. In contrast, haloperidol caused a prolonged (1. 5 to 12 h) decrease in the percentage of TH-IR neurons with FRA-IR nuclei in the VL-ARC. Haloperidol had no effect, however, on the overall number of TH-IR neurons in either of these regions. Co-administration of prolactin antisera (PRL-AB) blocked haloperidol-induced increases in both plasma prolactin concentrations and the percentage of TH-IR neurons expressing FRA in the DM-ARC, but had no effect on haloperidol-induced inhibition of FRA expression in TH-IR neurons in the VL-ARC. Intracerebroventricular (i.c.v.) administration of prolactin also increased the percentage of TH-IR neurons containing FRA-IR nuclei in the DM-ARC, but this effect was of longer duration (up to 6 h) than that of haloperidol in all but the most caudal portion of the DM-ARC. In the VL-ARC, prolactin caused a transient increase (at 1.5 h) in the percentage of TH-IR containing FRA-IR nuclei. These results demonstrate that prolactin regulates immediate early gene expression in TIDA neurons in male rats, and reveal that there are temporal differences in the responsiveness of discrete subpopulations of these neurons to prolactin. Prolactin causes a short-lived increase in FRA expression in TIDA neurons in the VL-ARC which is followed by a more prolonged activation of FRA expression in TIDA neurons in the DM-ARC.

Prolactin regulation of tuberoinfundibular dopaminergic neurons: immunoneutralization studies

The purpose of this study was to determine the effects of acute hypoprolactinemia on tuberoinfundibular dopamine (DA) neurons using a rabbit anti-rat prolactin antiserum (PRL-AB) to immunoneutralize circulating prolactin under basal conditions and at various times after haloperidol-induced hyperprolactinemia. The specificity of PRL-AB for prolactin was determined by examining the ability of unlabelled hormone to displace binding of 125I-labelled prolactin to PRL-AB. Tuberoinfundibular DA neuronal activity was estimated by measuring the concentrations of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence which contains terminals of these neurons. Systemic (i.v.) administration of 200 microl of PRL-AB decreased plasma prolactin concentrations below detectable levels for at least 4 h, and this was accompanied by a pronounced decrease in DOPAC concentrations in the median eminence of females, but not males. Central (i.c.v.) administration of 2 microl PRL-AB diluted up to 1:100 mimicked the inhibitory effect of systemic administration of PRL-AB on median eminence DOPAC concentrations suggesting that the tonic stimulatory effect of prolactin on the basal activity of tuberoinfundibular DA neurons in females occurs via a central site of action. In male rats, blockade of anterior pituitary DA receptors with haloperidol (1 mg/kg; s.c.) caused an prompt (by 1 h) increase in plasma prolactin concentrations which was maintained for at least 12 h. Haloperidol-induced hyperprolactinemia also caused a delayed (at 6 and 12 h) increase in median eminence DOPAC concentrations in these animals which was blocked by PRL-AB. Exposure of rats to initial priming periods of endogenous hyperprolactinemia of up to 6 h duration (followed by 6 h or more of PRL-AB-induced hypoprolactinemia) failed to alter median eminence DOPAC concentrations unless prolactin exposure was reinstated by an i.c.v. injection of prolactin. These results confirm that prolactin mediates the stimulatory effects of haloperidol on tuberoinfundibular DA neurons, and reveal that delayed induced activation of these neurons by prolactin is dependent upon a priming period of sustained hyperprolactinemia longer than 3 h for initiation and maintenance of this response.